Great question! Within our own solar system, the Earth is definitely unique. Then again, all of the planets are! Our Sun is host to four rocky ("terrestrial") planets -- three of which have large enough atmospheres to give them weather -- and four gas giants with very different ring systems. Each of these eight planets has its own distinct size, density, composition and orbit, and between them they have 140 known moons. Jupiter has the most (a whopping 63!), while Venus and Mercury are tied for the least -- none! I could go on and on about the qualities that make each planet special in turn, but in the interest of addressing the more time-sensitive aspect of the question, I'll leave it at this: The two factors that most set the Earth apart from its neighbors are the huge amounts of liquid water on its surface and, well, us! Scientists think that the single greatest requirement for life is the presence of liquid water, and the surface of our home planet is mostly just that -- 71% water!
Beyond our solar system, the question becomes a little more complicated. Perhaps the most popular astronomical project of our time is the search for planets orbiting stars other than the Sun, a.k.a. "exoplanets". As of today, there are 539 confirmed exoplanets, but we think from the first round of data from the Kepler spacecraft (currently in a solar orbit not unlike the Earth's) that there may be as many as 50 billion planets in our own galaxy -- the Milky Way -- alone... And the Milky Way is only one of 100 billion galaxies in the observable universe!
I'd imagine by this point you're looking at these numbers and thinking just how many planets like the Earth must be out there! It's true -- there are definitely plenty would-be Earth's just waiting to be found -- but there's a catch. The searches so far are best at picking up big planets close to their stars, where the orbit time is shortest. So most of the exoplanets we're finding have very little in common with the Earth. So most of the planets being found are very, very hot and not at all suitable for life.
Of the 539 confirmed exoplanets, only 20 are considered "Earth-like" so far, and none are smaller than 2-3 times the Earth's size. Allowing for the limitations of current observations, scientists think that as many as one in four Sun-like stars may have "Earth-like" planets orbiting them.
Think of it this way: There are many possible orbital distances, sizes and compositions. A very big, maybe infinite, universe is sure to have any of those possibilities play out many, many times.
The Earth is certainly unique in that it is a rare combination of characteristics.
Perhaps the story of Goldilocks and the three bears is the best comparison. Amid a whole bunch of other dramatically unsuitable planets, the Earth is that one that's "just right". But the more time we devote to studying the universe and sorting through the many newly-discovered exoplanets, the more likely we are to find other planets like ours.
And what if our search is misguided? Perhaps we'll find in the future that life can exist without water, or that extreme temperatures aren't as limiting as we thought. Either way, it seems a safe to say that the earth is very unusual-- most other planets are not "just right" for us.
I hope that answers your question! If you'd like to keep track of new exoplanet discoveries, bookmark this Kepler mission page and check back from time to time: http://kepler.nasa.gov/news/newsaboutplanetfinding/
- Becca (+mbw)
(published on 05/24/11)